Automated determination of distances is important for many applications. For example, the audio environment greatly affects the performance of a loudspeaker system, and e.g. the acoustics of a room significantly affect the spatial experience that can be provided by a spatial sound reproduction system. To provide the best possible spatial rendering it is therefore important that the sound reproduction system be properly calibrated to the specific audio environment. Since it is not possible in advance to account for every possible permutation of room size, sound system location and listener position etc, it is in practice only possible to provide a limited number of preset options. A proper calibration of a sound reproduction system must therefore be carried out with the equipment in situ. To optimize the sound reproduction, the room geometry, the listening position and the location of the rendering device should preferable be known. While it is possible to measure this data manually, this would represent an undesirable amount of effort on the part of the user, and would be subject to user error. If the room geometry can be measured automatically, optimization can be automatic and free from user error. Such a room geometry may be determined from distance measurements to room objects such as walls and therefore a practical system for determining such distances to objects is highly advantageous.
It has been proposed to determine distances to walls based on measurements of directional sound signals. WO200466673A1 discloses a system wherein a conventional loudspeaker array and at least one microphone are used to calibrate a sound bar system wherein a plurality of spatial channels are generated from a single loudspeaker device using a loudspeaker array and directed radiation of sound signals.
The disclosed system uses the conventional loudspeaker array to create a beam of directional sound which is aimed towards a wall. The reflected sound is recorded by a microphone and the time difference between emission of the sound and capture of the sound is used to determine the distance to the reflecting object. This approach is very similar to a standard sonar system. The use of a conventional loudspeaker array for this system had several disadvantages. A conventional loudspeaker array can only produce highly directional sound beams over a limited range of frequencies determined by the width of the array and the spacing of the loudspeakers. The limited bandwidth results in the disclosed system using a single frequency test tone for the calibration which may result in e.g. reduced signal to noise ratio compared to wider bandwidth signals. The disclosed system may also be prone to lobbing artefacts which can give rise to spurious results.
Another problem faced by the use of a conventional loudspeaker array is that the large aperture required for high directivity also results in a large audio beam spot size which reduces the resolution of the measurement system. To address this problem WO200466673A1 suggests that a focusing algorithm is used to reduce the beam spot size. However, in order to focus the beam, the distance to the reflecting object must be known in advance, i.e. it requires that the distance that is to be measured is already known. As such an iterative optimisation procedure is required to focus the beam to a suitable spot size and identify the location of the wall with a suitable accuracy. This is expensive in terms of both processing power and measurement/setup time.
Range detection systems based on ultrasound have also been used for determining the distance to walls. These systems radiate an ultrasound signal towards a wall and measure the time it takes before the ultrasound signal is received back. The distance may then be determined to correspond to half of the round trip delay for the ultrasound signal. However, such ultrasonic ranging systems require that the reflective surface is perpendicular to the ultrasonic sound beam and are very sensitive to deviations there from. Indeed, in many cases even a small deviation from a perpendicular angle results in the measured signals corresponding to a path with multiple reflections (e.g. of more walls) thereby leading to erroneous results and thus e.g. an erroneous calibration of a sound reproduction system. Sound ultrasonic ranging systems therefore tend to be impractical for many fixed calibration systems wherein the ranging device cannot practicably be positioned and aimed directly towards an object to which the distance is being measured. In particular, they tend to require manual operation and are unsuitable for many automated systems where the exact position and direction from the ranging device to the objects are not known.
Hence, an improved system for determining a distance would be advantageous and in particular a system allowing for increased flexibility, facilitated implementation, facilitated operation, improved accuracy, increased flexibility in geometric relationship between the system and object, improved suitability for automatic ranging and calibration, improved directionality, increased focussing, and/or improved performance would be advantageous.